LDL, or low-density lipoprotein, is a lipid transporter that transports lipids in the blood. LDL transports several kinds of lipids such as triglycerides and phospholipids; however, the main lipid transported by LDL is cholesterol. Recall that cholesterol is a 4-membered ring structure with a hydroxyl group attached to one of the rings. A decrease in LDL will decrease the amount of cholesterol carried by LDL which will, subsequently, increase the amount of free cholesterol found in blood; therefore, decreasing LDL will increase the amount of cholesterol found in the blood.

Prostaglandins are lipid molecules derived from arachidonic acid. Recall that arachidonic acid is synthesized from an essential fatty acid called omega-6 fatty acid; therefore, prostaglandins don’t depend on cholesterol and won’t be affected by decreased LDL levels. Heart attacks can often result from atherosclerosis, or thickening and clogging of artery walls due to build up of white blood cells (WBCs). The WBCs typically accumulate in arterial walls due to increased LDL levels; therefore, a decrease in LDL levels will decrease the risk of heart attack. Since lowered LDL levels decrease the amount of WBCs in arterial walls, there will be decreased inflammation. Recall that inflammation is caused by cytokine factors released by WBCs such as macrophages and granulocytes.

Fatty acids are long hydrocarbon chains that contain a carboxylic acid moiety on one end. Saturated fatty acids have no double bonds, whereas unsaturated fatty acids have one or more double bonds. Adding a double bond to a fatty acid will eliminate two hydrogen atoms. Knowing this information, we can deduce that a 15-carbon unsaturated fatty acid with one double bond will have two less hydrogen atoms than a saturated fatty acid of the same length. The molecular formula for the 15-carbon unsaturated fatty acid with one double bond will be , whereas the molecular formula for its saturated counterpart will be . Because of its two additional hydrogen atoms, the saturated fatty acid will have a greater molecular weight.

Geometric isomerism, or cis/trans isomerism, is characterized by the relative positions of functional groups around a double bond. A molecule is ‘cis’ if two identical functional groups are on the same side of the double bond. A molecule is ‘trans’ if the functional groups are on opposite sides. Only unsaturated fatty acids contain double bonds; therefore, only unsaturated fatty acids can participate in geometric isomerism.

A triglyceride has three fatty acids; this molecule has two (represented by the R chains). Phosphaditic acid is the simplest of the diacyl-glycerophospholipids; its phosphate group is bonded to only to the glycerol, and nothing else, which is not the case here. Among the three other choices, all of them accurately describe this molecule as a diacylglyceride phospholipid (hence the prefix phosphatidyl-); the phosphate is attached to serine as a head group, not a choline, nor ethanolamine, .

Incorrect answers are corrected below:

Most lipid double bonds are conjugated nonconjugated (typically three carbon atoms apart).

Lipids of the bilayer matrix most commonly move by transverse lateral diffusion (on same side of the membrane).

Proteins and carbohydrates embedded in the bilayer impart transverse symmetry asymmetry to the membrane.

An increase in the number of double bonds increases decreases the melting point of a triacylglyceride. 

Lipids are capable of storing the most energy per unit weight of any molecule. The complete oxidation of a fatty acid yields , compared to about  for carbohydrates and proteins. 

Triglycerides and cholesterol are not phospholipids, so they can be eliminated from the answer choices. Phosphatidylinositol and phosphatidylserine are present in cellular membranes, but they are typically less abundant than phosphatidylcholine, which is the correct answer. 

To answer this question, let's go through each of the answer choices to see what cholesterol's functions are.

It turns out that cholesterol is an important component of animal cell membranes. It helps to maintain both the structural integrity of the membrane, as well as its fluidity.

Cholesterol also serves as a precursor for all steroid hormones. In fact, if you look through all the steroid hormones, you'll find that they all have the characteristic 4-ring structure that cholesterol has.

Cholesterol also functions as a precursor for the production of bile salts in the liver. These bile salts are subsequently stored in the gallbladder and, when needed, released into the duodenum of the small intestine to aid in the digestion of lipids.

Cholesterol can deposit into the inner lining of blood vessels, however this is not a normal function of cholesterol. Rather, this is a pathological process that leads to a condition called atherosclerosis. This, in turn, can lead to the hardening of blood vessels, as well as contribute to the formation of blood clots that can impede the flow of blood in that vessel. These clots can also become dislodged and travel throughout the circulatory system, where it can become trapped in other blood vessels. This is a dangerous situation, because it can potentially lead to heart attack or stroke.

For animal cells at rest, the potential difference across plasma membranes is usually somewhere between  and . It is given by the Nernst equation:

Where  and  are constant,  is the ion's charge,  and  are the outside and inside concentrations respectively, and  is the temperature. Therefore, the potential difference decreases as the ion's charge increases, and is not independent of the concentration gradient. It increases -- it does not decrease -- with temperature.

Lipids with saturated tails are able to pack into very close proximity and form a relatively stable and solid bilayer because the fatty acids tails are straight and do not take up a lot of space. Adding cis-double bonds to these tails puts kinks in the them that make it harder for lipids to pack in close proximity. By unsaturating the tails of lipids in a bilayer membrane in this way, you can make that membrane more fluid.

An "amphipathic" molecule is one that has both hydrophilic and hydrophobic regions. A molecule that can act as both an acid and a base is amphoteric. Stereoisomers are molecules with the same molecular formula, but a different arrangement of atoms in space - an example is D-glucose and L-glucose.